1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a driving method thereof for automatically adjusting brightness of interference image displayed at ECB (Electrical Controlled Birefringence) sub pixels of cells formed in a quad type at a liquid crystal display panel in accordance with brightness of a background screen.
2. Discussion of the Related Art
A typical liquid crystal display employs a liquid crystal layer disposed between two substrates. In operation, an electric field is applied across the liquid crystal layer using opposing electrodes to controls the light transmittance of the liquid crystal layer to display a picture.
The above described liquid crystal display controls the light transmittance of individual liquid crystal cells in accordance with a video signal to display a picture. By using a liquid crystal display of an active matrix type employing active devices as switches, it is possible to realize a display capable of displaying moving pictures. In a typical liquid crystal display of the active matrix type, a switching device is provided for each crystal display cell. A thin film transistor (hereinafter, referred to as “TFT”) is commonly used as the switching device in liquid crystal display of the active matrix type as shown in FIG. 1.
Referring to FIG. 1, the liquid crystal display of the active matrix type converts a digital input data into an analog data voltage on the basis of a gamma reference voltage and supplies the analog data voltage to a data line DL. Concurrently, a gate pulse is supplied via a gate line GL to turn on the TFT to thereby charge a liquid crystal cell Clc with the data voltage on the data line DL.
A gate electrode of the TFT is connected to the gate line GL and a source electrode is connected to the data line DL. A drain electrode of the TFT is connected to a pixel electrode of the liquid crystal cell Clc and to an electrode of a storage capacitor Cst. A common electrode of the liquid crystal cell Clc is supplied with common voltage Vcom.
When the TFT is turned-on, the storage capacitor Cst charges a data voltage applied from the data line DL. The storage capacitor maintains a voltage of the liquid crystal cell Clc until a new voltage is charged to the liquid crystal cell Clc.
When the gate pulse is applied to the gate line GL, the TFT is turned-on to form a channel between the source electrode and the drain electrode, thereby supplying a voltage on the data line DL to the pixel electrode of the liquid crystal cell Clc. An electric field is generated between the pixel electrode and the common electrode. The electric field controls the arrangement of liquid crystal molecules of the liquid crystal cell Clc between the pixel electrode and the common electrode to modulate the transmission of light through the liquid crystal cell.
Liquid crystal displays having the above-described structure may be roughly classified into vertical electric field applying types and horizontal electric field applying types depending upon a direction of electric field used to drive the liquid crystal.
A liquid crystal display of vertical electric field applying type drives a liquid crystal using a vertical electric field (i.e. a field directed substantially perpendicular to the liquid crystal display panel surface) formed between a pixel electrode and a common electrode arranged in opposition to each other on upper and lower substrates. In a typical arrangement, the common electrode is on an upper substrate and the pixel electrode is on a lower substrate are each made of a transparent electrode so that the liquid crystal display panel has a large aperture ratio. However, a refractive index of the liquid crystal molecules is relatively large at a major axis direction thereof and a minor axis direction thereof compared to the index of refraction along other directions. Accordingly, when the liquid crystal is driven using a vertical electric field, there is a difference between a refractive index along a front view of the display at a front side and a refractive index as viewed along a side surface of the display. As a result, a viewing angle for the display is less than 90°.
In a liquid crystal display of horizontal electric field applying type, the liquid crystal is driven in an in-plane switching (hereinafter, referred to as “IPS”) mode using a horizontal electric field (i.e. a field directed substantially parallel to the liquid crystal display panel surface) between the pixel electrode and the common electrode arranged parallel to each other on the same lower substrate. In an IPS mode device, because the liquid crystal is driven by a horizontal electric field, there is substantially no difference between a refractive index as viewed from a position in front of the display and as viewed from a position towards the side of the display. As a result, the effective viewing angle is about 90°.
Typically, the liquid crystal cells of the liquid crystal display panel include RGB sub pixels of the stripe type. More recently, a liquid crystal display employing a liquid crystal display panel having cells of quad type has been developed to provide a liquid crystal display that may be selectively adjusted to have either a wide viewing angle or a narrow viewing angle. The cells of quad type are liquid crystal display panel may include one ECB (Electrical Controlled Birefringence) sub pixel and three RGB sub pixels.
FIG. 2 is a diagram showing a cell structure of quad type.
Referring to FIG. 2, a cell of quad type may include a R sub pixel, a G sub pixel, a B sub pixel, and an ECB sub pixel. The R and G sub pixels are arranged in parallel in an upper part of the cell, while the ECB and B sub pixels are arranged in parallel in the lower part of the cell.
The R and ECB sub pixels and the G and B sub pixels of a cell are not all connected to the same data line DL. In the illustrated example, the R sub pixel is located above the ECB sub pixel and the G and B sub pixels are arranged parallel to the R and ECB sub pixels, one above each other. As illustrated in FIG, 2, the R and ECB sub pixels are commonly connected to one data line DL, while the G and B sub pixels are commonly connected to another data line DL.
The R and G sub pixels and the ECB and B sub pixels of a liquid crystal cell are not all connected to the same gate line GL. In the illustrated case, the R sub pixel is horizontally adjacent to the G sub pixel and the G and B sub pixels are arranged parallel to the R and ECB sub pixels one above the other. Herein, the R and G sub pixels are commonly connected to one gate line GL and the ECB and B sub pixels are commonly connected to another gate line GL.
With the cell of a quad type connected to the data lines DL and the gate lines GL as described above, the number of data lines is decreased and the number of gate lines is increased when compared to the number of data lines and gate lines of a related art liquid display panel having a stripe type structure.
When the liquid crystal display panel is operated in a narrow viewing angle mode, the ECB sub pixels generate an image to interfere with the viewing of an image displayed by the RGB sub pixels from a position towards the side of the display.
A relationship between the display image generated by the RGB sub pixels and the interference image generated by the ECB sub pixels will be described with reference to FIG. 3.
Referring to 3, image (A) in FIG. 3 represents the display image to be displayed at the liquid crystal display panel. The image (A) is generated using the RGB sub pixels. Image (B) in FIG. 3 represents the interference image displayed using the ECB sub pixels.
The interference image (B) is displayed using the ECB sub pixels concurrently with the display of image (A) using the RGB sub pixels. When the display image and the interference image are simultaneously displayed, the display image of the mark (A) is visible and the interference image of the mark (B) is not visible when viewed from a viewing position towards the front of the liquid crystal display panel. On the other hand, when viewed from an angle along the side surface of the quad liquid crystal display panel, the display image is overlapped with the interference image in the view of an observer as indicated in image ‘C’ of FIG. 3.
A dark color image, for example, a character or other pattern, might be output on a bright colored background screen to be displayed the display image at the front side of the liquid crystal display panel. As a result, when the observer views the pixels from a position in front of the display, the interference image is not perceived. In other words, a display image of low brightness should be output on a background screen of high brightness to avoid perception of the displayed interference image while viewing the display image from a position in front of the liquid crystal display panel.
If a bright color display image is displayed on a dark color background screen, the display image is discerned to overlap with the interference image even when the observer views the pixels from an angle towards the front side of the liquid crystal display panel. In other words, if the display image of high brightness is output on the background screen of low brightness, the display image is perceived to overlap with the interference image even by an observer positioned directly in front of the liquid crystal display panel.